Some thoughts on cnc machine design principles. . .

[Ross77]

Ah, another thing. The X axis bearing close together bit. I was surprised at this myself but the maths points to it if:

Tell me more, I need to Know

My next machine, or future mod to this one, will put them closer together, but I know why I'm doing it and what I want to achieve. You'll have to decide if it works for your application. If you're not sure, go with Ross and space them out a bit.

thats the problem, that people look a designs and say "well he's done it like that and it worked" withouth understanding why.....still engineers tend to be belt and braces, think off a no. and double it (or is that the hourly rate?):whistling:

[routercnc]

OK, bit more about the close spacing of the X axis bearings.

If you run through the 'force_and_moments4' spreadsheet (post#1), you'll see the maths to support it. But there are design conditions:

1. You need to put the C of G of the gantry in the middle of the X axis bearings, i.e. a lean back style of gantry. This removes the moment on the bearings due to an offset C of G.

2. You need to arrange for the ballscrews (or whatever) to be placed at a set distance away in Z away from the side bearings, relative to the distance in Z to the tool from the side bearings. See formula at the bottom of tab3. This ends up being at a similar height to the tool, the slight difference being due to taking account of the friction in the bearings. In practice this would probably mean dual X axis ballscrews so you can run one down each side of the gantry. A single centre drive would get in the way of the cutter!
With this arrangement, at a fixed cutting height, there is no moment on the bearings during cutting.

3. You need to tend to cut at a similar height, i.e. thin materials. Once you cut at other heights, e.g. 3D, then the cutter is not always at the same distance from the bearing in Z, but the ballscrew is. This then starts to add a moment. You could get around this by raising and lowering the workpiece, which I have seen done, but adds complication.

4. You need to use modest accelerations, and modest gantry weights. In my example I used a 20kg gantry, and up to 2000mm/min2 acceleration. The gantry inertia was very small, and hardly added any moment. The main factors were the cutting force moment and the ballscrew force moment which basically cancelled to leave no moment.


With these conditions met, and no resulting moment, you are then left with the X axis bearings only supporting the weight of the gantry. They can then be close together or far apart and react the same load, so you can choose to mount them close together to gain travel. The danger of racking (made worse with closer bearings) is gone because you almost certainly need to drive the gantry with twin ballscrews to meet condition 2.

Hope this helps . . .

[Ross77]

Thanks for clarifying that, I understand the logic but I would say that it would be a very limited machine tho. Light weight , slow and the added expense of two ballscrews.

The cost for getting slightly longer rails is minimal compared to what you would need to do to counter the close beraings, also bear in mind that plain friction bearings require a 2:1 ratio anyway so they dont bind


The more I use my lathe im still amazed how even large chunks of steel can flex.
Just my 2 pennth again :naughty:

[Colin Barron]

Maybe this extra stiffness of the box section is why the likes of axyz and chinese makers use this design for the y axis?
Do they use aluminium or mild steel box section. I bet its steel, stronger and easy to weld plates onto it as well as better stiffness?
what is the main reason diy builders use aluminium, softer so easier to machine, lightness or does not rust?
Any ideas?
I am not American!

[jhobson]

I'm completely new to CNC and should be careful about posting in a 'sticky' thread but I didn't notice discussion about diagonal bracing for torsion. I don't mean to tech how to suck eggs.
Torsion results in diagonal forces so you want material on the diagonal. Where you see flat plates braced together with diagonal webs, then the webs greatly help with torsional rigidity - e.g you might see this in machine tool castings where it provides torsion stiffness but allows for simple moulding without need for cores. If your design lacks torsion stiffness then look to add material on the diagonal.
I remember this from some lecture almost 40 years ago. I also remember that it is much more complicated to calculate the stiffness compared to closed sections so a good reason to avoid in designs! I 'remember', but it might be an over-simplification, that non closed sections all have similar torsional stiffness as flat plate. So a tube with a slit down the length is a flat plate, not a circular tube, The detail of the end attachment may complicate this in reality but as a rule of thumb...